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Ostracoderms (lit. 'shell-skins') are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes and possibly the cyclostomes if anaspids are closer to them) and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.

An innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. Earlier chordates with gill precursors used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow-moving prey into their mouths.

Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had difficulty classifying them, as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeon, but later realized that they lacked movable jaws. Hence, he classified them in 1844 as a new group, named "ostracoderms" to mean 'shell-skinned' (from Greek ὄστρακον óstrakon + δέρμα dérma).

Ostracoderms have heads covered with a bony shield. They are among the earliest creatures with bony heads. The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth-like structures." Neil Shubin writes: "Cut the bone of the skull open…pop it under a microscope and…you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull--one of the earliest in the fossil record--is made entirely of little teeth. Teeth originally arose to bite creatures (see Conodonts); later a version of teeth was used in a new way to protect them."

Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.

It was long assumed that pteraspidomorphs and thelodonts were the only ostracoderms with paired nostrils, while the other groups have just a single median nostril. It has since been revealed that even if galeaspidans have just one external opening, it has two internal nasal organs.

After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed, as they coexisted without noticeable decline for about 30 million years.

The Subclass Ostracodermi has been placed in the division Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.

Major groups

Major groups of ostracoderms
Group	Class	Image	Description
Cephalaspido- morphi	Cephalaspidomorphi or cephalaspids ('head-shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day.
	Galeaspida (extinct)		Galeaspida ('helmet-shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.
	Pituriaspida (extinct)		Pituriaspida ('pituri-shields') are a small group of extinct armoured jawless fishes with tremendous nose-like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia.
	Osteostraci (extinct)		Osteostraci ('bony-shells') lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian. Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to lampreys than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates. They are thought to be the sister-group of pituriaspids. Together, these two taxa of jawless vertebrates are the sister-group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone.
Other groups	Other groups
	Pteraspido- morphi (extinct)		Pteraspidomorphi ('wing-shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida.
	Thelodonti (extinct)		Thelodonti ('feeble-teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit-feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free-swimming organisms.
	Anaspida (extinct)		Anaspida ('no-shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.

See also

• Acanthodii
• Placoderm

References

1. Benton, Michael (2009) Vertebrate Palaeontology Edition 3, page 44, John Wiley & Sons. ISBN 9781405144490.
2. Walker; Liem (1994). Functional Anatomy of the Vertebrates An Evolutionary Perspective (2 ed.). Sanders College Publishing. ISBN 0-03-096846-1. "Gills are not present in the pharyngeal pouches of protochordates as they are in fishes; rather the slits of protochordates are part of their feeding mechanism." - p 32 "Water is drawn into the pharynx ... The pharynx wall is perforated by many vertically elongated, pharyngeal slits ... Cells in the endostyle of the pharyngeal floor secrete mucus that entraps minute food particles." - p 35 "Amphioxus also gains oxygen and discharges carbon dioxide from the water flowing through the pharynx even though gills are not present." - p 35
3. Maisey, John G. (1996). Discovering Fossil Fishes (illustrated ed.). New York: Henry Holt & Company. p. 37. ISBN 9780805043662.
4. ^ Shubin, Neil (2009). Your Inner Fish: A Journay into the 3.5 Billion Year History of the Human Body (reprint ed.). New York: Pantheon Books. pp. 85–86. ISBN 9780307277459.
5. "Fossil fish reveals how jaws evolved - Planet Earth Online". Archived from the original on 2012-07-03. Retrieved 2014-04-11.
6. "GEOL 331 Invertebrate Paleontology". Archived from the original on 2016-10-20. Retrieved 2014-04-11.
7. Vertebrate jaw design locked down early
8. Morales, Edwin H. Colbert, Michael (1991). Evolution of the vertebrates : a history of the backboned animals through time (4th ed.). New York: Wiley-Liss. ISBN 978-0-471-85074-8.{{cite book}}: CS1 maint: multiple names: authors list (link)
9. Dead fish breathes new life into the evolutionary origin of fins and limbs
10. Sansom, R. S. (2009). "Phylogeny, classification and character polarity of the Osteostraci (Vertebrata)". Journal of Systematic Palaeontology. 7: 95–115. doi:10.1017/S1477201908002551. S2CID 85924210.
11. Turner, S.; Tarling, D. H. (1982). "Thelodont and other agnathan distributions as tests of Lower Paleozoic continental reconstructions". Palaeogeography, Palaeoclimatology, Palaeoecology. 39 (3–4): 295–311. Bibcode:1982PPP....39..295T. doi:10.1016/0031-0182(82)90027-X.
12. Sarjeant, William Antony S.; L. B. Halstead (1995). Vertebrate fossils and the evolution of scientific concepts: writings in tribute to Beverly Halstead. ISBN 978-2-88124-996-9.
13. Donoghue, P. C., P. L. Forey & R. J. Aldridge (2000). "Conodont affinity and chordate phylogeny". Biological Reviews of the Cambridge Philosophical Society. 75 (2): 191–251. doi:10.1111/j.1469-185X.1999.tb00045.x. PMID 10881388. S2CID 22803015.{{cite journal}}: CS1 maint: multiple names: authors list (link)
14. Turner, S. (1999). "Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance". In A. J. Boucot; J. Lawson (eds.). Palaeocommunities, International Geological Correlation Programme 53, Project Ecostratigraphy, Final Report. Cambridge University Press. pp. 42–78.
15. The early and mid Silurian. See Kazlev, M.A., White, T. (March 6, 2001). "Thelodonti". Palaeos.com. Archived from the original on October 28, 2007. Retrieved October 30, 2007.{{cite web}}: CS1 maint: multiple names: authors list (link)
16. Ahlberg, Per Erik (2001). Major events in early vertebrate evolution: palaeontology, phylogeny, genetics, and development. Washington, DC: Taylor & Francis. p. 188. ISBN 0-415-23370-4.
17. Hall, Brian Keith; Hanken, James (1993). The Skull. Chicago: University of Chicago Press. p. 131. ISBN 0-226-31568-1.

v t e Evolution of fish
Fish Timeline of fish evolution
Forerunners	Basal member †Pikaia Cephalochordate †Cathaymyrus Lancelet Olfactores †Haikouella Tunicate †Myllokunmingiidae? (†Haikouichthys, †Myllokunmingia) †Zhongxiniscus?
Jawless fish	Cyclostomata Hagfish Hyperoartia Lamprey †Conodonts †Protoconodonta? †Paraconodontida †Conodonta †Ostracoderms †Pteraspidomorphi †Thelodonti †Anaspida †Cephalaspidomorphi †Galeaspida †Pituriaspida †Osteostraci
Jawed fish	†Placoderms †Antiarchi †Arthrodira †Brindabellaspida †Petalichthyida †Phyllolepida †Ptyctodontida †Rhenanida †Acanthothoraci †Pseudopetalichthyida? †Stensioellida? †Acanthodii †Acanthodiformes †"Climatiiformes" †Diplacanthiformes †Ischnacanthiformes Cartilaginous Elasmobranchii †Ctenacanthiformes †Hybodontiformes †Phoebodontiformes †Xenacanthida Ray Shark Holocephali †Eugeneodontida †Iniopterygiformes †Petalodontiformes †Symmoriiformes Chimaera Bony Lobe-finned †Onychodontiformes Coelacanth Rhipidistia †Porolepiformes Lungfish Tetrapodomorpha Ray-finned †"Palaeonisciformes" Cladistia Polypteriformes Chondrostei Acipenseriformes Neopterygii †Pycnodontiformes Halecomorphi Ginglymodi Teleost
Lists	Lists of prehistoric fish spiny sharks placoderms cartilaginous bony lobe-finned List of transitional fossils
Related	Prehistoric life Transitional fossils Vertebrate paleontology
† extinct

• Paleozoic jawless fish
• Paraphyletic groups
• Polyphyletic groups